1. Field of the Invention
The present invention relates to a coating composition, and particularly to a coating composition which is composed of a silicone resin solution containing boron nitride, a glaze and an inorganic pigment in certain specific proportions and which has excellent adhesion, lubrication and release properties.
2. Description of the Prior Art
Boron nitride (BN) exhibits excellent electric insulation and heat conduction properties even at a high temperature and yet its thermal expansion coefficient is small. Thus, it is durable against heat shock, and is hardly wetted or eroded by molten metal, molten salt or molten glass. Accordingly, it is useful, for instance, as a releasing agent for glass molding or die casting.
As a method for using it as a releasing agent, there may be mentioned (1) a method wherein a BN shaped product or powder is used by itself, or (2) a method wherein a solution obtained by dispersing a BN powder in water, an oil or a solvent, optionally with an addition of a binder, is coated on a substrate, followed by baking. However, the BN shaped product in the method (1) is so expensive that the use of such a shaped product is not practical. Whereas, when a BN powder is directly coated on the substrate, its adhesion will be poor, and it is impossible to obtain a coating film with good quality. On the other hand, when a solution obtained by dispersing the BN powder according to the method (2) is baked onto the substrate at a high temperature of a level of about 700.degree. C., peeling or cracking is likely to result in the coating film. Therefore, such a solution is hardly useful as a mold releasing agent for molding glass at a temperature of about 700.degree. C. Even when the baking is conducted at a temperature lower than 700.degree. C., the temperature raising or cooling operations are required to be conducted under mild conditions. Otherwise, cracks or blisters are likely to result. Thus, the method (2) has a drawback that the baking operation is time consuming.